This invention relates to an intake air flow meter for internal combustion engines of automobiles and particularly to a heat sensing wire type air flow meter.
There are various methods of measuring the volume of the intake air drawn into an internal combustion engine. Among these, the heat sensing wire type air flow meter is most widely used since it has a good response and is capable of measuring the mass of air drawn in so that pressure correction is not necessary. This kind of air flow meter comprises a platinum wire of 70 to 100 .mu.m in diameter which is strained in the air horn as proposed in the patent application of Japanese Patent Publication No. 48893/1974 (U.S. Pat. No. 3,747,577), the Laid-Open No. 19227/1972 and the Laid-Open No. 64134/1976. The flow meters in these applications, however, have a problem of durability and especially have other problems that the detecting portion is likely to be damaged by the backfiring caused when the engine is not running in good condition.
In order to solve these problems, as improved air flow meter has been proposed in which the detecting portion is comprised by winding a platinum wire on a supporting body such as hollow body made of ceramic material and having a coating material thereon, whereby the air flow meter has improved mechanical strength (Japanese Patent Applications Nos. 53-42547 and 53-65748).
The detecting portion above, however, is likely to be affected by heat transmitted from a member supporting the detecting portion, since the ratio of length to diameter of the platinum wire, of which the detecting portion is comprised, is comparatively small and the detecting portion is supported by lead wires whose portions exposed in an air flow passage are not small enough in diameter to prevent the heat from being transmitted.
Accordingly, the output signal of the air flow meter is affected by the wall temperature of the air horn.